Transcript 9.2 - 4ubiology

Nerve cell membrane
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Electrochemical message is created by the movement of ions
across the nerve cell membrane
The resting nerve membrane has a electrical potential
difference (potential) of -70 mV due to an unequal
concentration of positive ions across the membrane
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When the nerve is excited there is a rapid reversal in the
potential of the membrane
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More positive ions outside of the membrane
Resting potential = -70 mV
Becomes +40 mV
Called the action potential
The movement of the action potential through an axon
conducts the neural impulse (message)
Resting Potential
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High concentration of K+ ions inside cell
High concentration of Na+ ions outside cell
K+ ions diffuse in and Na+ diffuse out
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But cell more permeable to K+
The net result is that relatively more +ve ions end up outside
the neuron
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so the outside is more positive with respect to the inside
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This establishes electrochemical gradient for the ions
across the membrane
this charge difference is responsible for the resting
potential.
At this point the neuron is said to be polarized.
K+ diffuses
out faster
than Na+
diffuses in;
membrane
is said to be
polarized
The action potential
[1] Resting potential: neuron is polarized at -70 mV.
[2] Upon excitation Na+ gated channel proteins open
(due to a change in shape of the protein itself: makes the
membrane more permeable to Na+ ions now) which
allows Na+ ions to diffuse into the neuron down the
electrochemical diffusion gradient.
[3] This causes the inside of the neuron to become
increasingly more positive: this is depolarization and
neuron has become depolarized
The action potential
[4] Na+ channels close and K+ gated channels now
open
[5] K+ ions diffuse out of the neurone down the
electrochemical diffusion gradient, so making the inside
of the neuron less positive (= more negative) again:
this is repolarization and the neuron has become
repolarized
[6] The neurone has its resting potential restored.
During this time the Na+ and K+ ions which have
diffused in/out of the cell are redistributed by active
transport (sodium-potassium pump)
Na-K pump
ion channel protein
Refractory Period
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Nerves conducting an impulse cannot be
activated until resting membrane is
restored
 Must
happen before next action potential can
be conducted
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Time required for repolarization to happen
is call the refractory period
 Last
between 1 to 10 ms
Movement of Action Potential
Threshold Levels & the
All-or-None Response
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A potential stimulus must be above a
critical value (threshold level) to
produce a response.
Threshold Levels & the
All-or-None Response
Increasing the intensity of the stimuli
above threshold will not produce an
increased response.
 Intensity of impulse & speed of
transmission remain the same.
 Known as the all-or-none response.
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 Neurons
either fire maximally or not at all.
Threshold Levels & the
All-or-None Response
Differentiating Between Warm & Hot
 The more intense the stimulus, the
greater the frequency of impulses.
 Intense stimuli excite more neurons.
 Different
neurons will have different
threshold levels.
 This affects the number of impulses
reaching the brain.
Synaptic Transmission
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Spaces between two neurons or a neuron
& an effector is called a synapse.
Synaptic vesicles containing
neurotransmitters (NTs) found in the end
plates of axons.
Impulse down axon  NTs released from
presynaptic neuron  NTs diffuse across
synaptic cleft  depolarizes postsynaptic
neuron.
Synaptic Transmission
Motor end plate
Synapses between motor nerve and muscle
Note: this is not a
physical junction, there
is actually a small gap
of approx 20 nm
between the cells so
there is no membrane
continuity so nerve
impulses cannot cross
directly.
synaptic vesicles
pre-synaptic membrane
post-synaptic membrane
Types of synapse
Excitory synapses
 Binding of neurotransmitter to postsynaptic
neurone
 opens Na+ gated channels
  Na+ diffuses IN
 depolarisation
  action potentials
 so nerve impulses can continue around the
nerve circuit.
Types of synapse
Inhibitory synapses
 Binding of neurotransmitter to postsynaptic
neurone opens K+ gated channels
  K+ diffuses OUT
  inside of neurone becomes even more – ve
and so impossible to depolarize
  no action potentials
 so nerve impulses cannot continue around the
nerve circuit.
Neurotransmitters
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Acetylcholine acts as an excitatory NT by opening
Na+ channels on postsynaptic neuron, causing
depolarization.
Cholinesterase (from postsynaptic neuron)
destroys acetylcholine preventing a constant state of
depolarization.
Inhibitory NTs make the postsynaptic membrane
more permeable to K+.
 Neuron
becomes hyperpolarized.
 More Na channels must be opened to depolarize and
get an action potential
Neurones are
connected together
(normally via axons
and dendrites) at
synapses
Summation
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Effect produced by the accumulation of
NTs from two or more neurons.